Early prior art isolation systems involved intricate positioning of tools which were installed down-hole after the gravel pack. These systems are exemplified by a commercial system which at one time was available from Baker. This system utilized an anchor assembly which was run into the wellbore after the gravel pack. The anchor assembly was released by a shearing action, and subsequently latched into position.
Certain disadvantages have been identified with the systems of the prior art. For example, prior conventional isolation systems have had to be installed after the gravel pack, thus requiring greater time and extra trips to install the isolation assemblies. Also, prior systems have involved the use of fluid loss control pills after gravel pack installation, and have required the use of thru-tubing perforation or mechanical opening of a wireline sliding sleeve to access alternate or primary producing zones. In addition, the installation of prior systems within the wellbore require more time consuming methods with less flexibility and reliability than a system which is installed at the surface.
Later prior art isolation systems provided an isolation sleeve which was installed inside the production screen at the surface and thereafter controlled in the wellbore by means of an inner service string. For example, as shown in U.S. Pat. No. 5,865,251, incorporated herein by reference, illustrates an isolation assembly which comprises a production screen, an isolation pipe mounted to the interior of the production screen, the isolation pipe being sealed with the production screen at proximal and distal ends, and a sleeve movably coupled with the isolation pipe. The isolation pipe defines at least one port and the sleeve defines at least one aperture, so that the sleeve has an open position with the aperture of the sleeve in fluid communication with the port in the isolation pipe. When the sleeve is in the open position, it permits fluid passage between the exterior of the screen and the interior of the isolation pipe. The sleeve also has a closed position with the aperture of the sleeve not in fluid communication with the port of the isolation pipe. When the sleeve is in the closed position, it prevents fluid passage between the exterior of the screen and the interior of the isolation pipe. The isolation system also has a complementary service string and shifting tool useful in combination with the isolation string. The service string has a washpipe that extends from the string to a position below the sleeve of the isolation string, wherein the washpipe has a shifting tool at the end. When the completion operations are finalized, the washpipe is pulled up through the sleeve. As the service string is removed from the wellbore, the shifting tool at the end of the washpipe automatically moves the sleeve to the closed position. This isolates the production zone during the time that the service string is tripped out of the well and the production seal assembly is run into the well.
Prior art systems that do not isolate the formation between tool trips suffer significant fluid losses Those prior art systems that close an isolation valve with a mechanical shifting tool at the end of a washpipe prevent fluid loss. However, the extension of the washpipe through the isolation valve presents a potential failure point. For example, the washpipe may become lodged in the isolation string below the isolation valve due to debris or settled sand particles. Also, the shifting tool may improperly mate with the isolation valve and become lodged therein.
Therefore, a need remains for an isolation system for well control purposes and for wellbore fluid loss control which combines simplicity, reliability, safety and economy, while also affording flexibility in use. A need remains for an isolation system which does not require a washpipe with a shifting tool for isolation valve closure.